For a laser drilling machine which performs drilling, for example, in a fabrication process of a printed circuit board, a positioning control system is required to sequentially irradiate a laser beam to plural positions of a work under drilling. A steerable mirror control system is often used to materialize a high drilling throughput and high-accuracy drilling.
In general, a laser drilling machine is a numerical control (NC) machine having a hierarchical control structure, and a steerable mirror control system is included in its lowest level hierarchy. In a control system of high level hierarchy (hereinafter called “the high-level control unit”), the order of drilling is optimized based on CAM (Computer Aided Manufacturing) data for a printed circuit board such that a high drilling throughput can be materialized, and perforation position coordinates are written in an NC program in an order of the drilling.
Such an NC program has been prepared beforehand and, when drilling begins, the high-level control unit subjects perforation position coordinates in the program successively to a coordinate transformation and transmits time-series angle command data to the steerable mirror control system. For the drilling of each perforation in the form of a true circle, there is a need to irradiate a laser beam after a steerable mirror has been brought into a standstill at an angle designated by an angle command data. The transmission of the angle command data and control of the irradiation of the laser beam are, therefore, performed in unison within the high-level control unit.
Principal elements of the steerable mirror control system include a steerable mirror as a movable body, a rotary actuator for changing the position or angle of the steerable mirror over a range of approx. ±15° max., and a control circuit for feedback controlling the angle of the steerable mirror.
As the rotary actuator, an electromagnetic actuator is used in common. This electromagnetic actuator generates a drive torque according to an electromagnetic principle. The steerable mirror is fixedly secured on a drive shaft of the rotary actuator so that the drive shaft serves as a support for the steerable mirror. On this drive shaft, a sensor and a moving coil or moving magnet are also arranged in addition to the steerable mirror.
The angle of a rotation of the steerable mirror is detected by the sensor, and the detected angle data is fed to a feedback control circuit. The feedback control circuit is realized by an analog control unit constructed of an operational amplifier or a digital control firmware constructed in combination of a microprocessor and a program. A single positioning operation of the steerable mirror may vary from the order of 0.01° to approx. 30° in terms of angular stroke, and the positioning time may range from less than 1 millisecond to several milliseconds (ms).
The steerable mirror control system receives a single angle command data as a step input signal to perform a single positioning operation. Namely, the steerable mirror control system rotates the steerable mirror on the basis of the thus-received single angle command data. As soon as the steerable mirror begins a rotary motion, an integral compensation functions to bring the resulting mirror angle into conformity with the angle command data without error. In this compensation, a value obtained by subtracting each detected angle data from the angle command data, that is, a tracking error signal is integrated. For the assurance of a stable operation of the feedback loop in the steerable mirror control system, there is also a need to set a phase margin and gain margin of a loop transfer function at sufficiently large values. Differentiation of detected angle data or use of a so-called state observer makes it possible to apply a stabilizing compensation and a phase lead compensation, both of which rely upon angular velocity signals. These control methods are known well as fundamentals of feedback control theories [see KATAYAMA, Toru, “Fundamentals of Feedback Control” (in Japanese), Chapter 6 to Chapter 7, Asakura-shoten, Ltd., Tokyo (May 20, 1987)].
With a view to shortening the time required to position a steerable mirror, a technique is also employed to make a feedback loop operable at a wide band of frequencies. Described specifically, in the case of the above-mentioned electromagnetic actuator, the steerable mirror, sensor and the like arranged on the drive shaft act as inertial loads, and therefore, shaft torsional vibrations may occur in a high-speed operation. As plural torsional vibration modes generally exist in a range of several kHz or more, the feedback loop is made applicable over a broad frequency range by a vibration-mode stabilizing compensator. This stabilization compensator serves to estimate the state values of the torsional vibration modes and to feedback the estimated values (see, JP-A-2002-40357 and JP-A-2002-40358).
Another technique is also known (see JP-A-2003-57570). According to this technique, a strain sensor is arranged to detect torsional vibrations of a drive shaft, and signals from the strain sensor are used to reduce torsional vibrations.
A feedback loop is constructed by combining these methods with the above-mentioned integral compensation and phase lead compensation. Characteristics of the feedback loop are adjusted such that the time required for each single positioning operation (positioning time) meets a specified target and an overshoot and residual torsional vibrations, both of which are contained in a transient response (settling response) in the neighborhood of a target angle, fall within their corresponding permissible ranges.
A Fourier analysis of a series of angle command data transmitted in time sequence from the high-level control unit (hereinafter called “angle command pattern”) makes it possible to understand that each angle command pattern has its own different frequency spectrum. If any certain spectrum component coincides with a resonance frequency of the above-mentioned torsional vibrations in this case, its vibration mode is considered to result in the production of residual vibrations so that the positional accuracy of drilling would be deteriorated.
With a view to overcoming the above-mentioned problem, a further technique has been proposed (see JP-A-2000-28955). With respect to three successive angle command data, an operating frequency for a steerable mirror is calculated based on their positioning times. If this operating frequency is determined to be a condition that tends to induce residual vibrations, the third positioning time is extended to operate the steerable mirror such that a resonance frequency range can be avoided, thereby realizing to perform successive positioning operations at higher speed and higher accuracy.
In addition, a hard disk drive can be mentioned as an apparatus making use of a high-speed and high-accuracy, positioning control technique. To record and read digital information on the surface of a magnetic disk, the hard disk drive is equipped with a feedback control means for driving the magnetic head at high speed and positioning it with high accuracy. The control means has a controller specialized for high-speed drive and another controller specialized for high-accuracy positioning, and makes use of mode switching control that alternately switches a high-speed drive mode and a high-accuracy positioning mode. The switching of the control modes is performed during the drive of the magnetic head so that, in the controller for the mode to which the control mode has been switched, a state variable at the moment of the switching, in other words, a response derived from the initial state value significantly affects a transient response of the positioning operation. A compensation signal corresponding to the initial state value is, therefore, inputted on or after the time of the mode switching to improve the transient response [see JP-A-8-137551, and YAMAGUCHI, Takashi and HIRAI, Hiromu, “Design of Initial Value Compensation with Additional Input for Mode Switching Control System and Its Application to Magnetic Disk Drives” (in Japanese), SICE Transactions, 32(8), 1219-1225 (1996)].
To make an improvement in the throughput of a laser drilling machine, there is a tendency to shorten the time interval of step signals in an angle command pattern (hereinafter called “command interval”). A shorter command interval, however, leads to the inclusion of more high-frequency components in the frequency spectrum of an angle command pattern, so that due to high-order vibration modes of a drive shaft, residual vibrations tend to occur, resulting in a concern about potential deterioration in the positioning accuracy. Nonetheless, the above-described conventional techniques indicate no means for the realization of high-speed and high-accuracy positioning without extending the positioning time for various angle command patterns.
To make the command interval extremely short, it is necessary to perform irradiation of a laser beam and to begin the next positioning operation before the steerable mirror comes to a complete standstill subsequent to an entrance into a permissible range of settling response amplitude. If any settling response of the preceding positioning is still remaining at this stage, the state values in the dynamic characteristic modes contained in the feedback loop (hereinafter called“initial state value”) are not zero (0) at the time point of the start of the next operation.
An angle command pattern is generally irregular, and therefore, can take any one of various initial state values. Especially in a positioning operation over a small angle stroke, the positioning time is short so that the steerable mirror is brought close to a target angle before the influence of the initial state values attenuates sufficiently. As a consequence, the subsequent settling responses have different waveforms, respectively. When a demand arises for drilling of still higher accuracy in the future and the permissible range of settling response amplitude becomes narrower accordingly, a technique will be required to control the settling response amplitude small for any angle command pattern.
The conventional techniques, however, shows no means for stably controlling small the settling responses for positioning, which begin from initial state values that can vary widely.
In feedback control of a hard disk drive, a controlled variable is a positioning error of the magnetic head to the center of a target data track, so that subsequent to the switching of the control modes to a high-accuracy positioning mode, the target value is always zero (0), i.e., a constant value. A steerable mirror control system, on the other hand, is different in that an angle command pattern as a target value successively varies like step signals. They are also different from each other in that the former includes the switching of the control mode. The above-described conventional technique which is directed to hard disk drives indicates no means for stably controlling settling responses small in such control as tracking a target value, which successively varies like step signals, by a single control mode.